This application claims priority to EP 01300167.2, herein incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates generally to lithographic projection apparatus and more particularly to lithographic projection apparatus including a contaminant barrier.
2. Background of the Related Art
Generally, lithographic projection apparatus in accordance with the present invention include a radiation system for supplying a projection beam of radiation, a support structure for supporting patterning structure, the patterning structure serving to pattern the projection beam according to a desired pattern a substrate table for holding a substrate and a projection system for projecting the patterned beam onto a target portion of the substrate.
The term xe2x80x9cpatterning structurexe2x80x9d as here employed should be broadly interpreted as referring to means that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term xe2x80x9clight valvexe2x80x9d can also be used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning structure include:
A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
A programmable mirror array. One example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuation means. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors; in this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronic means. In both of the situations described hereabove, the patterning structure can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT patent applications WO 98/38597 and WO 98/33096, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning structure as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning structure may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
In a lithographic apparatus the size of features that can be imaged onto the substrate is limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices, and hence higher operating speeds, it is desirable to be able to image smaller features. While most current lithographic projection apparatus employ ultraviolet light generated by mercury lamps or excimer lasers, it has been proposed to use shorter wavelength radiation of around 13 nm. Such radiation is termed extreme ultraviolet (EUV) or soft x-ray and possible sources include, for instance, laser-produced plasma sources, discharge plasma sources, or synchrotron radiation from electron storage rings.
Some extreme ultraviolet sources, especially plasma sources, emit substantial amounts of contaminant molecules, ions and other (fast) particles. If such particles are allowed to reach the illumination system, which is of course downstream of the radiation source, or further downstream in the apparatus they can damage the delicate reflectors and other elements and cause build-up of absorbing layers on the surfaces of optical elements. Such damage and built-up layers cause an undesirable loss of beam intensity, increasing necessary exposure times and hence reducing machine through-put, and can be difficult to remove or repair. To prevent contaminant particles reaching the illumination system, it has been proposed to provide a physical barrier or window in the exit to the radiation system or the entrance to the illumination system. However, such a window is itself prone to damage from the contaminant particles and build-up of absorbing layers. Also, since most materials absorb extreme ultraviolet radiation at the wavelengths proposed for use in lithography, the window even when new and clean will absorb a significant proportion of the beam energy, reducing throughput. This absorption can cause thermal stress in the window even leading to breakage of the window.
EP-A-0 957 402 discloses a contamination barrier which uses a hollow tube, positioned between the last solid surface of the projection system and the substrate, and flushed with gas flowing towards the substrate to prevent contaminants emitted from the resist being deposited on the projection lens.
One aspect of embodiments of the present invention provides a contaminant barrier that may be used in a lithographic projection apparatus to remove undesirable contaminants, e.g. produced by a radiation source.
This and other aspects are achieved according to the invention in a lithographic apparatus as specified above, including a contaminant barrier comprising ionization means for ionizing a gas provided in a region traversed by said projection beam.
The ionization means can be, for example, an electron source or a plasma generated by capacitive or inductive RF discharge or ac discharge.
In one embodiment of the invention, getter plates are provided upstream (the terms upstream and downstream are used herein to indicate direction relative to the direction of propagation of the projection beam, unless the context otherwise requires) of the ionization means. The ionized gas and contaminants are attracted to the getter plates, which are negatively charged, and thus removal of the contaminants is enhanced. Such contaminants may, for example, be ions and charged particles emitted by the radiation source along with the desired radiation forming the projection beam. The ionization effect can be improved by providing a magnetic trap to trap free electrons downstream of the purge gas supply.
In a further preferred embodiment a plasma generated is confined to a tube having a greater length than width. Because the ions generated in the plasma have much greater masses than the electrons, their temperature will be much lower than that of the electrons. As the diffusion rate of particles is governed by their temperature the electrons will rapidly diffuse out of the plasma. Because of the length to width ratio of the tube in which the plasma is confined, electrons will preferentially move towards the walls of the tube than towards the ends of the tube. The deficiency of electrons in the plasma volume creates a charge polarization, causing the ions, both those of the source and those of the plasma, to follow the electrons out of the plasma towards the walls of the tube and become trapped. This ambipolar diffusion therefore aids removal of contaminants from the projection beam. The contaminant barrier is thus able to protect effectively the delicate optics of the illumination and projection systems from the particles emitted by the radiation source.
In a further embodiment of the present invention, the apparatus further comprises gas supply means to generate a flow of purge gas in a region traversed by the projection beam, said purge gas flow being substantially directed in the opposite direction to the direction of propagation of the projection. Because the gas flow in the contaminant barrier is largely against the direction of propagation beam, said purge gas effectively impedes the contaminants travelling with the projection beam.
The contaminant barrier of the present invention many be used, to advantage, with the contamination barrier of EP-A-0 957 402, mentioned above.
According to a further aspect of the invention there is provided a device manufacturing method comprising:
providing a substrate that is at least partially covered by a layer of radiation-sensitive material;
providing a projection beam of radiation using a radiation system;
using patterning structure to endow the projection beam with a pattern in its cross-section;
projecting the patterned beam of radiation onto a target portion of the layer of radiation-sensitive material, and ionizing a gas in a region traversed by the projection beam.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range 5-20 nm).